My new server runs qmail, which I've never used before. There is no inbound mail on the server (all the clients have mail hosted elsewhere, but some systems on the server send mail. I'm about to move a client there who has several parked domains, and looking at the smtp log, the server is already blocking many attempts to relay spam using one of the parked domain names (probably because a related domain is already hosted there).
How do I ensure those mails stay blocked, while allowing legitimate addresses to send out?
Server OS is CentOS and hosting software is Kloxo.